Guide vanes for axial fans

ABSTRACT

A guide vane arrangement for axial fans, intended to translate the rotational component of the gas flow velocity after passage through the impeller (1) into a substantially axial velocity, including a ring (2) of guide vanes disposed downstream of the fan and in spaced relationship therewith. Alternate guide vanes are axially displaced with respect to the remaining ones, so that alternate guide vanes are at a first axial distance from the fan and the remainder are at a second axial distance from the fan. As an alternative, alternate guide vanes have a portion of the end part facing towards the fan removed, so that the forward edge of alternate guide vanes is at a first distance from the fan and the remaining guide vanes are at a second axial distance, and the guide vanes are non-uniformly distributed along the periphery of the ring.

TECHNICAL FIELD

The following invention relates to a guide vane arrangement for axialfans, intended to convert the rotational component in the gas flowvelocity after passage through the impeller into a substantially axialvelocity, the arrangement including a ring of guide vanes disposeddownstream of the fan and in spaced relationship therewith.

BACKGROUND ART

The gas flow downstream of the impeller of an axial fan normallyrotates. The rotational energy can be translated into useful energy by aguide vane arrangement on the outlet side of the fan, this arrangementconverting the rotational velocity to an axial velocity component. Thepressure is raised in this way, and the efficiency of the fan increases.Normally, however, the sound level also increases at the same time.

The sound from a fan comprises tonal components, i.e. tones withdiscrete frequencies and a wide band noise with a continuous frequencyspectrum. Considerable efforts have been made primarily to lower thetonal components, by a suitable arrangement and embodiment of guidevanes on the output side of the fan.

It is accordingly a general understanding that the noise generateddecreases with increasing distance between the impeller and guide vanes,see M. J. Benzakein, J. Acoust. Soc. Am. 51 (1972), 1427-1438 and W.Neise, Proc. INTER-NOISE 1988, pp 767-776. It has been found, however,this is not always applicable.

A guide vane is described in the Swedish patent 8802136-5, which hasimproved aerodynamic and acoustic properties.

It has also been found earlier that a non-uniform distribution of theguide vanes in the ring of guide vanes can give rise to certain acousticimprovements, although it has been found that large deviations from auniform distribution of the guide vanes give rise to aerodynamicproblems.

It is also known that certain acoustic characteristics can be improvedby a portion being cut out from alternate guide vanes in their theforward portions.

FIG. 1 illustrates how the strength in an individual tone can be reducedby displacing the guide vanes in the circumferential direction of theguide vane ring. It will also be seen from the same figure that thenoise at higher frequencies over about 500 Hz also increases at the sametime. The measurement has been made for a fan R.P.M. of n=970 and adisplacement of alternate guide vanes of β=10° and β=0°.

DISCLOSURE OF THE INVENTION

The object of the present invention is to lower individual, disturbingtones in the fan sound, as well as lower the general noise level in asimple way.

The first-mentioned object is achieved with a guide vane ring of thekind mentioned in the introduction, and with characteristics disclosedhereinbelow 1.

The second object is achieved by a further development of the inventiveguide vane arrangement, in which alternate guide vanes are axiallydisplaced relative the remaining ones, so that alternate guide vanes areat a first axial distance from the fan, and the remainder at a secondaxial distance, simultaneously as the guide vanes are non-uniformlydistributed in the guide vane ring circumference. Preferably, alternateguide vanes are displaced in the circumferential direction relative tothe remaining guide vanes with a constant displacement so that thedistance in the circumferential direction between juxtaposed guide vanesalternates between two given values. This arrangement reduces bothindividual tones and the general wide band noise from the fan. Inaddition, this combination of axial displacement and circumferentialdisplacement of the guide vanes provides improved efficiency of the fan,compared with the case using a guide vane arrangement with only axialdisplacement of the guide vanes, or only rotation of the vanes in thecircumferential direction.

By the combination of measures according to this further development ofthe apparatus in accordance with the invention, there are achievedhighly important advantages, both with respect to acoustics andefficiency, considerably exceeding the effects achieved by theindividual measures of axial displacement or circumferential rotation ofthe guide vanes.

In accordance with another advantageous embodiment of the inventiveapparatus, the axial displacement of the guide vanes is in the interval0,4-0,7 l/l_(ch), preferably 0,5 l/l_(ch), where l denotes the magnitudeof the displacement and l_(ch) the length along the guide vane.

In accordance with yet another advantageous embodiment, the displacementin the circumferential direction is in the interval 5°-15°, and ispreferably 10°.

In accordance with a further advantageous embodiment of the apparatus inaccordance with the invention, the guide vane ring arrangement isdivided into two rings axially in tandem, alternate guide vanes beingcarried by one ring and the other guide vanes by the other ring, therings being axially displaceable relative to each other or radiallyrotatable relative each other about a common axis. It is thus possibleto adjust the guide vane arrangement in a simple way to achieve optimumconditions.

In accordance with a further advantageous embodiment of the invention,in the portion facing towards the impeller the guide vanes are designedwith a web configuration between the radially outer and inner portionsof the guide vane such that the arcuate length along the single-curvedguide vane at the level of the web is shorter than at said mentionedouter and inner portions.

BRIEF DESCRIPTION OF THE DRAWINGS

Emodiments of the arrangements in accordance with the invention,selected as examples, will now be described in greater detail and withreference to the accompanying drawings, wherein

FIG. 1 illustrates the effect on the sound level from the fan whenalternate guide vanes are displaced in the circumferential direction inaccordance with prior art;

FIG. 2 illustrates an axial fan with guide vanes arranged downstream ofthe fan;

FIG. 3 schematically illustrates five different guide vane arrangementswhich include: a) guide vanes arranged uniformly according to the priorart, b) alternate guide vanes displaced in the circumferential directionof the guide vane ring according to the prior art, c) alternate guidevanes axially displaced in accordance with the invention, d) acombination of axial displacement and rotation of the guide vane ring ina circumferential direction in accordance with the invention, e) thecombination of the displacement in the circumferential direction and theprovision of a cut-out in alternate guide vanes in accordance with theinvention;

FIG. 4 illustrates the effect on the sound from the fan resulting fromthe axial displacement of the alternate guide vanes in accordance withthe operational case c) in FIG. 3;

FIG. 5 illustrates the effect on the sound level from the fan of thecombination of axial displacement and displacement of the guide vanes inthe circumferential direction of the guide vane ring according to theoperational case d) in FIG. 3

FIG. 6 illustrates the reduction of sound in the tone at the bladefrequency as a function of the rotation in the circumferential directionfor a given axial displacement of the guide vanes;

FIG. 7 illustrates an embodiment of the arrangement where two guide vanerings are used and

FIG. 8 illustrates a particular guide vane configuration.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 illustrates an axial fan installed in a duct 4 and having a guidevane ring 2 arranged in spaced relationship with, and downstream of theimpeller 1. The number of guide vanes is preferably between 0.5 and 2.1times the number of blades in the impeller.

In FIG. 1 the effects on the sound from a fan with guide vanearrangements according to the principles shown in FIGS. 3a and 3b arecompared under the conditions given above. It will be seen from FIG. 1that displacement Lw (dB) of alternate guide vanes in thecircumferential direction of the guide vane ring according to FIG. 3bresults in a heavy decrease of a tone at the blade frequency f (Hz) of160 Hz, while there is an increase in the noise level for frequenciesover about 500 Hz.

FIG. 4 compares the effect on the sound level f (Hz) of the guide vanearrangements according to FIGS. 3a and 3c. It will be seer that there isa considerable lowering of the tone at the blade frequency f (Hz) 160 Hzof about 3.5 dB, while there is an increase o the wide band highfrequency noise over about 500 Hz. The operational conditions are thesame as for FIG. 1.

In FIG. 5 the sound levels for the guide vane arrangements according toFIGS. 3a and 3d are compared. A surprising result will be seen from thisfigure in that the combination of axial displacement and rotation (i.e.,being out of alignment) in the circumferential direction of the ring ofalternate guide vanes leads to a reduction of the sound level over theentire frequency range, compared with a conventional guide vanearrangement with uniformly distributed guide vanes. FIG. 5 demonstratesthat both discrete tones at lower frequencies and the wide band highfrequency noise are antenuated in an embodiment according to FIG. 3d.The axial displacement of the guide vanes is l/l_(ch) =0,5, where ldenotes the magnitude of the displacement and l_(ch) the length alongthe guide vane, i.e. the displacement amounts to half the length of theguide vane. Rotation in the circumferential direction amounts to 10°.The curves are measured for a fan R.P.M. of n=970.

In FIG. 6 there is illustrated the attenuation ΔL_(W) at the bladefrequency, as a function of the rotation B of alternate guide vanes in aguide vane arrangement where the alternate guide vane is also axiallydiplaced by half the length of the guide vane. The graphs are shown fortwo different revolutionary velocities of the fan, namely n=970 rpm, andn=1430 rpm. This diagram shows that a considerable reduction of tonalcomponents is achieved in the angle range 5°-15° for the axialdisplacement of the guide vanes that is under consideration.Measurements have also shown that some improvement in aerodynamicefficiency of the fan in the region where the best sound attenuation isachieved.

Another acoustically advantageous embodiment of the apparatus accordingto the invention is illustrated in FIG. 3e. In this embodiment a portionof the end part facing towards the impeller of alternate guide vanes isconcave, so that the forward edge of these guide vanes is at a firstaxial distance from the impeller and the remainder at a second axialdistance. In addition, the guide vanes are non-uniformly distributedround the circumference of the ring.

In FIG. 7 there is illustrated a further advantageous embodiment of theguide vane ring 2 in the apparatus in accordance with the invention. Thering 2 includes two rings 20,22 mounted on a common shaft. Here,alternate guide vanes are carried by one of the two rings and the otherby the other ring. The two rings are mutually axially displacable andrelatively rotatable about the common shaft. With this embodiment of theguide vane ring 2 there is enabled in a simple way the axialdisplacement and rotation along the circumference of the guide vanesrelative each other so that desired properties are achieved.

FIG. 8 illustrates an embodiment of a guide vane 6, which is found to beadvantageous in the arrangement according to the invention. The endportion of the guide vane 10, which is intended to face towards theimpeller has an edge 10 with a parabola-like configuration, so thatbetween the inner and outer longitudinal edges 12 and 14 of the guidevane 6 there is a web with a shorter length L₂ along the guide vane thansaid edges L₁ and L₃, respectively. The guide vane has a straight backedge 16. If the height of the web from the inner longitudinal edge 12 isdenoted by H₁ and the total height of the guide vane by H₂ the positionof the web is determined by the condition:

    0.4<H.sub.1 /H.sub.2 <0.9,

and preferably

    0.5<H.sub.1 /H.sub.2 <0.8

We claim:
 1. Guide vane arrangement of axial fans, the arrangementtranslating the rotational component of the gas flow velocity afterpassage through the impeller (1) into a substantially axial velocity,which comprises:a ring (2) of guide vanes arranged downstream of theimpeller and in spaced relationship therewith, wherein alternate guidevanes with an edge facing the impeller are axially displaced relative toremaining guide vanes and wherein alternate guide vanes are located afirst axial distance from the fan and the remaining guide vanes arelocated at a second axial distance from the fan, and the guide vanes arenon-uniformly distributed along a circumference of the ring. 2.Arrangement according to claim 1, wherein a portion of an end partfacing towards the impeller (1) of the alternate guide vanes is concaveso that the forward edge of said guide vanes is at a first axialdistance from the fan and the remainder of the guide vanes is at saidsecond axial distance from the fan.
 3. Arrangement as claimed in claim2, wherein the alternate guide vanes are out of alignment in thecircumferential direction relative the remaining guide vanes, with aconstant displacement, so that the distance in the circumferentialdirection between adjacent guide vanes varies between two given values.4. Arrangement as claimed in claims 1 or 2, wherein the axialdisplacement between the alternate guide vanes and the remaining guidevanes is in an interval of l/l_(ch) =0.40-0.7 where l denotes themagnitude of the displacement and l_(ch) denotes the length of the guidevane.
 5. Arrangement as claimed in claim 2, wherein the angulardisplacement in a circumferential direction has an interval of 5°-15°.6. Arrangement as claimed in claim 1, wherein the guide vane ring (2)comprises first and second rings (20, 22) positioned axially in tandemwherein said alternate guide vanes are carried by said first rings andthe remaining guide vanes by said second ring, and wherein said firstand second rings are mutually axially displaceable and/or radiallyrotatable relative each other about a common axis.
 7. Arrangement asclaimed in claim 1, wherein the guide vanes, in a part facing towardsthe fan, are formed with a web between radially outer and inner portionsof the guide vanes, and wherein an arcuate length at the level of theweb is shorter than at said outer and inner portions.
 8. Arrangement asclaimed in claim 1, wherein the number of guide vanes is between 0.5 and2.1 times the number of blades of the impeller.
 9. Arrangement asclaimed in claim 4, wherein l/l_(ch) =0.5.
 10. Arrangement as claimed inclaim 3, wherein the angular displacement in the circumferentialdirection has an interval of 10°.
 11. Arrangement as claimed in claim 4,wherein the angular displacement in the circumferential direction has aninterval of 10°.